The present invention is directed to using laser bonding to connect two electrical members together which are gold, gold coated or gold alloy coated. In particular the present invention is advantageous in inner lead bonding of a tape automated bonding (TAB) tape to the electrical bumps on an integrated circuit die, such as semiconductors. Thermal compression bonding, the current industry standard for inner lead bonding, uses 15,000 psi pressures and 400.degree. C. temperatures which would have a damaging effect if leads were bonded to bonding pads or bumps coated over semiconductor structures, especially as the bonding pads become more miniaturized. Further, thermal compression bonding is commonly optimized to one specific integrated circuit type and different sized circuits require a different set of tooling which is time consuming to replace and re-optimize. In addition the thermode must be cleaned periodically resulting in decreased system throughput. Thermosonic bonding is also well known, but suffers the drawback that the ultrasonic energy can damage the materials, the speed is limited, and the resolution may not be sufficient for closely pitched bonds.
It is well known that highly reflective metals are difficult to laser bond since laser irradiation occurs in the visible and near-visible regions of the spectrum. For CO.sub.2 lasers, this reflectivity is traditionally overcome by coating the metals to be joined with an organic material, such as flux, that absorbs the energy and transfers the heat via thermal conduction. The use of additional coating that must be removed after bonding is undesirable, particularly on devices that have close pitches which makes thorough cleaning difficult. Failure to clean residual organics can degrade the reliability of the circuit. Several solutions have been proposed. U.S. Pat. No. 4,023,005 discloses a method of welding highly reflective metallic members wherein one member is coated with a metal skin of nickel or palladium with low reflectivity to enable welding by a laser. The metal skin is chosen which will not vaporize and as the molten alloy of the metallic members cools a weld nugget alloy is formed containing metal from the metal skin as well as the metallic members. Likewise, U.S. Pat. No. 4,697,061 discloses a method for laser welding a highly reflective covering to a base layer wherein both the covering and the base layer are covered with a metal skin of solder that is less highly reflective of the laser. Each of these prior art techniques, however, suffers the drawback that intermetallics containing the low reflectivity metal and the highly reflective metals occur at the bond interface. At this location these intermetallics can cause reliability problems, especially if the bond is subject to thermal cycling, thermal shock, or mechanical shock. In the case of copper/gold bonds, a tin coating may form brittle intermetallic compounds throughout the entire bond interface.
Furthermore, there are no known satisfactory microelectronic laser bonding techniques for bonding gold, gold coated or gold alloy coated electrical members. Gold-to-gold bonds can be fabricated with thermal compression or thermosonic bonding instruments, but with the drawbacks as previously described. Microelectronic laser bonding typically employes coating tin on a member prior to bonding in order to absorb sufficient laser energy to fuse or weld a bond; see, for instance, U.S. Pat. No. 4,697,061 entitled "Method For Welding By Means of Laser Light." However the use of tin coatings can severly limit shelf life due to drawbacks like tin whiskers, phase changes, oxidation, and development of a reaction layer which consumes free tin. These tin drawbacks are particularly applicable to copper and copper base alloy substrates due to interdiffusion and formation of copper-tin compounds. As a copper substrate ages in storage, the thickness and uniformity of free tin can be reduced as free tin is replaced by a duplex layer of Cu.sub.6 Sn.sub.5 and Cu.sub.3 Sn. This duplex layer is less absorptive of laser energy than free tin and can thus degrade subsequent laser bonding. Likewise, it has been found that electrodeposited tin maintained at less than 13.degree. C. will transform from the white (beta) tin of tetragonal form to the gray (alpha) tin of cubic centered form. As a result the specific gravity of the tin is lowered from about 7.3 to about 5.75 and the metallic properties of the tin are destroyed. As "tin pest" develops, a loose tin powder is formed which can easily separate from the base metal. The base metal accordingly becomes susceptible to the effects of corrosion.
Furthermore, tin-whisker metal filaments sometimes grow spontaneously from tin coatings which can form electrical shorts across electrical conductors with fine line definitions. The mechanism of tin whisker growth is not well understood, and the filaments may grow within days or several years after the tin is coated. Several approaches to control tin whisker growth have been suggested, such as short storage times, whisker inhibiting additions to the tin coating solutions, and reducing the amount of hydrogen absorbed in the plating metal by ultrasonic agitation of the plating solution and/or alternating the polarity of the electrodes during plating. Gold-tin eutectic solder has many desirable attributes, such as good wetting of gold plated surfaces, high tensile strength, good fillet formation, long shelf life, high resistance to chemical attack and corrosion, and a relatively wide tolerance for temperature and pressure variations. Moreover, the negative aspects of tin coating such as tin pest and whisker growth do not appear. Thus, a gold-tin solder may have the advantageous properties of pure gold-to-gold bonding.
In view of the above, there exists a need for laser bonding microelectronic electrical members such as high density TAB tape leads to pads or bumps on integrated circuits or substrates in a gold-to-gold contact environment that overcomes the drawbacks commonly associated with tin coatings.